The present invention, in some embodiments thereof, relates to a tensioning mechanism for a textile feed to a stepped operation digital textile printer and, more particularly, but not exclusively, to such a textile printer that prints rolls of textile fabric.
Digital printers generally use a stepped feed. The material to be printed is advanced to a new position, the feed is stopped and the printer head prints the newly exposed material.
Stepped feeds have been used for printing on paper and like materials for many years. However, when feeding rolls of fabric, a problem arises in that the fabric overfeeds and thus becomes loose. Loose fabric is difficult to print on since part of the substrate may be hidden under a fold, and in any case, if the fabric is not taut, then the print will be uneven and distorted.
Fabric is fed along and adheres by a press roller to a ‘tacky’ conveyor belt. The feeding action is done by the conveyor belt that keeps pulling in new fabric as glued fabric moves tautly under the printheads. Then, after printing, the fabric is pulled away from the belt. The feeding action in digital machines is done in accurate steps. In each step the printing carriage prints across the fabric.
If wrinkles form on the ‘tacky’ belt, they can collide with the printheads, causing damage both to the printheads and the printing process.
In greater detail, there are woven fabrics that suffer from uneven internal tension from each edge of the fabric towards the center. Progressive increase in tension from each edge of the fabric towards the center portion is caused by progressive increase in the lengths of the threads from the center portion towards each edge. When a fabric of this character is fed into the press roller, slack may accumulate at the edges below the press roller. The slack may bunch and eventually create wrinkles that then pass the press roller.
Dealing with this common fabric quality issue is done by feeding the fabric to the press roller at higher tension. The increased tension stretches the slack fabric at the sides and thus may prevent the bunching phenomenon.
Increasing tension at the input to the press roller is accomplished by adding resistance to the fabric's motion created by the pulling of the ‘tacky belt’.
Fabric resistance to the belt's pulling action is commonly provided in the course of digital printing. In most cases, the fabric is fed through a roll that resists spin due to a slip-clutch coupled to its shaft or by transferring the fabric through two round static bars creating high friction due to sharp wrapping angles.
These methods rely on building tension when the fabric is in motion and have no ability to contribute required tension between steps when no pulling action is carried out.
However, the stepped feed in digital printing machines makes it difficult to continuously maintain stable tension because deceleration and stopping of the press roller is not correlated with inertia of the fabric's motion.
Woven fabrics are usually not stretchable and for this reason they are more sensitive to small tension loss after every step. The tension loss may cause bunching of slack fabric at the sides.